Rotational clutches are frequently used as one of the mechanisms for engaging or disengaging the various gear components of a transmission in order to establish different gear ratios between an input member and an output member. A conventional rotational clutch assembly typically includes a set of clutch plates and a set of friction discs, sometimes referred to as a clutch pack, interleaved between one another in a clutch housing. When the clutch assembly is disengaged, the clutch plates and friction discs normally turn past one another without contact. However, when the corresponding components of a particular clutch, i.e., a drive member and a driven member, are to be engaged during a particular gear range, for example, a hydraulically actuated or spring-loaded piston forces the clutch plates and friction discs together. Friction surfaces on the clutch plates and the friction discs interact until the drive member and the driven member of the clutch assembly rotate in unison without slip.
In operation, a great deal of thermal energy is generated during the engagement and disengagement of the clutch plates and the friction discs, as well as during the period of full engagement, when the kinetic energy generated by the engaged clutch pack is also translated into a large amount of thermal energy. This thermal energy must be dissipated to prevent damage to the various components of the clutch assembly, particularly the frictional surfaces of the clutch plates and the friction discs. A continuous supply of a coolant, such as transmission fluid, is typically supplied to the clutch housing to serve this purpose. In a rotational clutch assembly, the transmission fluid may be supplied to an inside diameter portion of the engaged clutch plates and the friction discs and allowed to flow by centrifugal force across the plate surfaces to an outside diameter portion. The hot transmission fluid is then directed away from the clutch assembly to pass through a heat exchange process for transfer and release of the thermal energy absorbed into the transmission fluid.
When the rotational clutch assembly is not engaged, the clutch plates and the friction discs simply rotate past one another without contact. During this period of disengagement, the amount of thermal energy that must be dissipated is minimal. Furthermore, simply maintaining a continuous flow of transmission fluid to the clutch pack during disengagement may also result in significant inefficiencies. For example, depending on the relative speed of the rotating drive member with respect to the disengaged, driven member, drag losses may be generated as a result of shear experienced by the transmission fluid between the clutch plates and the friction discs. The shear increases proportionally with the amount of transmission fluid provided to the clutch pack during disengagement. Thus, particularly in gears where the relative rotational speed differential between the clutch plates and the friction discs is highest, it is desirable to limit the flow of coolant to the clutch pack.
Various clutch cooling systems have been proposed to address controlling the flow of coolant to the clutch pack during engagement and disengagement. For example, U.S. Pat. No. 5,988,335 describes actively controlling the flow to the clutch with a diverter valve and a sensor arrangement to sense the gear ratio of the transmission and divert flow from the clutch assembly in response to the transmission being in a selected gear ratio. U.S. Pat. No. 6,244,407 proposes a more passive system that does not rely on a sensor actuated valve. Rather, an outer ring is mounted onto the piston used to actuate engagement of the clutch pack. The outer ring has an orifice provided therein for allowing a flow of coolant therethrough. The outer ring is movable between a first position wherein the orifice is closed and the drive and driven members are disconnected and a second position where the orifice is open to allow the flow of pressurized fluid through the orifice to the clutch pack when dictated by movement of the piston to engage the clutch plates and the friction discs. Other types of “slider valve” arrangements are common in the industry, wherein the piston moves the slider valve in a direction to uncover an orifice for increasing coolant flow to the clutch pack during engagement. Typically, a spring, for example, may be employed to close the slider valve over the orifice when the clutch disengages.
As described above, conventional clutch cooling systems can often be complex and/or require the addition of various components to provide a variable flow of coolant to the clutch pack. The increased complexity of these designs may add to the cost of manufacture, assembly, and maintenance of the transmission and creates additional opportunities for failure during operation. As such, a clutch cooling system is needed that eliminates the requirement for additional components while taking advantage of the natural operational characteristics of rotational clutch assemblies.